1. Field of the Invention
The invention relates generally to the field of temperature measurement and specifically to the field of pyroelectric infrared radiation detection.
2. Description of the Related Art
Noncontact temperature measurements are possible through the detection of infrared radiation. Many different detectors are available to convert infrared radiation into an electrical signal. There are two main types of detector, thermal and quantum. Thermal detectors operate by a change in the detector's electrical characteristics caused by the infrared radiation directly heating the material. This type of detector has wavelength independent response and can be operated without cooling. Quantum detectors offer an increase in sensitivity over thermal detectors as well as faster response times, but require cooling to liquid nitrogen or helium temperatures.
There are four main types of thermal detector, thermopile, pneumatic, thermistor and pyroelectric. Thermopile detectors operate on the same principle as do thermocouples in which voltage output is produced by the Seebeck effect. That is, a voltage potential is produced by two dissimilar metals that are connected and heated. These detectors have good sensitivity and a DC response, but suffer from slow response times. The DC response allows for static temperature measurements. A pneumatic detector utilizes the expansion of a gas as it is heated. The gas chamber forms the dielectric for a capacitor which undergoes a change in capacitance as the gas expands. These detectors also give good sensitivity and a DC response but they lack durability. Thermistor detectors operate on the principle that resistance is a function of temperature, as the thermistor heats up its resistance changes giving a relatively fast response time and a DC response but they suffer from a lower sensitivity. In a pyroelectric detector, incident infrared radiation causes a change in polarization in the pyroelectric material producing a voltage. These detectors give fast response times and good sensitivity but lack a DC response. It is known that through use of a mechanical chopper it is possible to obtain a DC response using a pyroelectric detector. The mechanical method however also limits the detector's response time.
Pyroelectric detectors have been used to make static measurements in the human body, Fraden (U.S. Pat. No. 4,797,840) describes a noncontact device for measuring the tympanic temperature. This patent is herein incorporated by reference. Fraden uses pyroelectric materials in a passive manner.